Polyamides have been employed as a useful resin for a considerable time. In particular, polytetramethyleneadipamide, hereinafter referred to as nylon 46, is a useful polyamide that demonstrates heat resistance, toughness and chemical resistance. These properties are important in compounds used as structural materials. Since nylon 46 also exhibits good self-lubricity in comparison to other resins, nylon 46 can be used in sliding members such as bearings, gears, and other mechanical parts requiring high abrasion resistance.
Recently, plastic sliding members have been used extensively in bearings that are severely abraded; in non-lubricated systems under heavy load; in bushings used at elevated temperatures; or in sliding members having a small wall thickness. The performance tolerances of plastic sliding members operating under such conditions have become more stringent.
In general, in applying a plastic to sliding members such as bearings, it is desirable to select a plastic that possesses suitable mechanical properties, such as rigidity, creep resistance, and resistance to heat at deformation temperatures and at continuous operating temperatures. In addition, the plastic should possess suitable sliding properties, such as a low dynamic friction coefficient, a high critical pressure velocity a small abrasion loss and a low tendency to abrade a metal in frictional contact with the plastic.
A polyamide resin, and in particular, nylon 46, demonstrates excellent mechanical properties and excellent heat resistance. However, nylon 46 has not demonstrated the suitable sliding properties, such as low friction and high abrasion resistance, that are required in sliding members. Therefore, in order to improve sliding properties of a polyamide resin, like nylon 46 for example, Japanese Patent Provisional Publication No. 62-185747 proposed a method of adding polytetrafluoroethylene and potassium titanate to nylon 46.
However, as equipment performance requirements have increased, downsizing and reduction in wall thickness of the sliding members also have increased. In particular, the demand for thin-walled sliding members has increased. Nylon 46 can exhibit improved sliding properties by the above-mentioned polytetrafluoroethylene and potassium titanate treatment, but the resin nevertheless exhibits poor fluidity in the molten state compared to other polyamides. Fluidity in the molten state is important because fluidity helps allow the downsizing and the wall-thickness reduction of the sliding member. Therefore, it would be desirable to provide a polyamide resin composition that demonstrates excellent mechanical properties, and that also demonstrates a sufficient fluidity in the molten state.
In general, previous methods for imparting fluidity to the molten resin included adding a hydrocarbon, like a paraffin or a polyethylene wax; a metallic soap, like calcium stearate or lead stearate; a long-chain carboxylic acid, like stearic acid; an amide compound, like amidopalmitic acid or ethylenebisstearylamide; an ester compound like stearic acid monoglyceride or cetyl palmitate; an alcohol, like mannitol or stearyl alcohol; or a polymer, like a polyethylene, a polypropylene or a polyethylene oxide.
However, the addition of a low molecular weight compound, such as a hydrocarbon, a metallic soap, a long-chain carboxylic acid, an amide compound, an ester compound or an alcohol, adversely affects the surface appearance and mechanical properties of the polyamide. The addition of a polymer, such as polyethylene or polypropylene, leads to a phase separation due to a poor compatability between the polymer and the polyamide, and also adversely affects the mechanical properties and heat resistance of the polyamide. Furthermore, because the thermal decomposition temperature of polyethylene oxide is about 200.degree. C., polyethylene oxide cannot be added to a polyamide having a molding temperature of about 200.degree. C. or greater.